I currently use a Black Diamond (30L don't remember model name atm) backpack. It has served me well for 4 1/2 years. I use smaller stuff sack type bags inside to organize loose edc gear.
I always have a few lights and spare cells, SAK (except airports etc.
), pocket mask, moderate first aid kit, spare socks, silk longsleeve undershirt, beanie hat, warm gloves, large bandanna, sunglasses, thermal blanket, rain poncho, water tablets, compass, mirror, firestarter, matches, paper and pen, paracord, caribiners, powdered Gatorade, powerbar type food, water bottle, cell phone, couple bucks in change, wallet, checkbook, keys. I don't like getting caught unprepared.
For me nothing beats a backpack for ease of mobility. If you're carrying 20+ pounds of gear a shoulder bag gets uncomfortable, especially after a couple miles. If you have to run, climb, jump, flip, ride, ski, or any other sort of high energy physical activity the backpack stays with your body much better; staying out of your way and not shifting your CG very much.
For agility and durability nothing beats a quality ski/ mountaineering backpack.
(This will prob get moved from the LED section since it isn't about LED lights by the way)